The present invention relates to a new process for producing a fiber-reinforced thermoplastic resin molded product, which is suitable for producing automobile parts, electrical appliances, industrial materials, civil engineering materials, commodity products, etc. These products are required to have good outer appearance as well as high mechanical strength and high rigidity.
The present invention also relates to a product such as a tiptoe core for a safety shoe, manufactured by said new process.
As a process for manufacturing a thermoplastic resin molded product reinforced with fibers, it is known that a sheet of continuous fibers or a chopped glass fiber mat impregnated with a molten thermoplastic resin (hereafter referred to as GMT), is used. GMT is reheat-melted and press-molded after having been cut out in order to adapt it to the shape of an objective product.
However, when GMT is molded, the resulting molded product has deficiency in mechanical strength and outer appearance, due to insufficient impregnation of a thermoplastic resin to glass fibers. Further, when GMT is heat-melted, a phenomenon tends to occur that it expands in a direction of the thickness, which will lower a heat conduction to the inside of the GMT to result in low heat efficiency
In order to solve the problems caused by the insufficient impregnation with the resin, JP-A-7-164439 discloses a molded sheet as a material for a hot stamping molding and a high speed press-molding. Such a molded sheet is prepared by scattering and accumulating fiber-reinforced thermoplastic resin base material, followed by molding it under heat and pressure.
Moreover, JP-A-7-184704 and JP-A-7-11-56410 disclose a tiptoe core for a safety shoe molded by using such a molded sheet mentioned above.
However, the manufacturing process of a molded product disclosed in JP-A-7-164439 requires three heating steps i.e. a step of preparing the fiber-reinforced thermoplastic resin base material, a step of preparing a sheet and a step of re-heating the sheet for press-molding. This process is not preferred from a view point of the energy consumption and causes the heat deterioration of the thermoplastic resin used.
Further, the step of preparing a sheet leads usually to a trim-loss which will lower the yield of the material obtained. Moreover, in the press-molding, a cutting out (a blanking) of the sheet is required in order to adapt the size of the sheet to a mould, wherein a blanking-loss occurs. Such will further lower the work efficiency as well as the production efficiency.
As a result, a tiptoe core for a safety shoe disclosed in JP-A-7-184704 and JP-A-7-11-56410 can manage to satisfy a mechanical strength for L kind and S kind stipulated in Japanese Industrial Standard (JIS) T 8101 (a safety shoes made of leather). However, the molding by using the above mentioned sheet tends to result in the poor productivity and economy.
Under these circumstances, it is an object of the present invention to provide a new process for producing a fiber-reinforced thermoplastic resin molded product with high mechanical strength and no substantial heat deterioration of the resin used, which is attained by an efficient heating without applying shear stress to the fiber-reinforced thermoplastic resin base material.
It is another object of the present invention to provide a fiber-reinforced thermoplastic resin molded product such as a tiptoe core for a safety shoe, produced by said new process.
A process for producing a fiber-reinforced thermoplastic resin molded product of the present invention to accomplish the above objects comprises a step of supplying a base material wherein a fiber-reinforced thermoplastic resin base material in a string form or in a tape form is scattered and accumulated, a step of forming a molten mass wherein a heating gas is passed to said base material accumulated to heat-melt it thereby to form a molten mass, and a molding step of wherein said molten mass is supplied to a mold and then is press-molded to form a molded product.
According to the present invention, the fiber-reinforced thermoplastic resin base material in a string form or in a tape form is scattered and accumulated and a heating gas is passed to openings or gaps among said bulkily accumulated base materials, whereby said base material can be quickly and uniformly heat-melted to form a molten mass. Thus resulting molten mass can be press-molded to produce a molded product with superior mechanical strength attributed to the long residue fibers contained and good outer appearance. Further, in the present invention, after said base material having been produced, the heat-melting of the resin can be one time, whereby the cost of energy consumption will be reduced and the work efficiency will be improved as well as the heat deterioration of the resin will be lowered.
In the present invention, when a bulk density of the fiber-reinforced thermoplastic resin base material in the accumulated state is defined as xcfx813, and a true density of the base material is defined as xcfx810, it is preferred to satisfy the following formula.
{fraction (1/100)}xe2x89xa6xcfx811/xcfx810xe2x89xa6xc2xd
When the base material satisfies the above formula, the accumulation of said base material can become properly bulky, whereby heating gas can efficiently pass through the base material and thus the heating can carried out in a short period of time. The above bulk density xcfx811 is the density of the base material which is scattered and accumulated randomly as much as possible in a vessel having a inner diameter bigger than the length of the base material. The above true density xcfx810 the density (a theoretical density) is the density of the base material itself.
Further, the fiber-reinforced thermoplastic resin base material used in the present invention has preferably the following characteristics of from a) to d).
a) a string form with an average diameter of from 0.1 to 1.5 mm,
b) a reinforcing fiber content of from 15 to 80 vol %,
c) a average length L of from 10 to 50 mm,
d) L/D of from 15 to 100, where D is an average diameter.
When the fiber-reinforced thermoplastic resin base material has the above characteristics, the base material become a fine and a needle-like shape to make the resulting accumulated base material easily voluminous, whereby the heating gas can be smoothly passed through the base material to result in a uniform and rapid heating of the base material.
Further, after passing the heating gas to the accumulated fiber-reinforced thermoplastic resin base material to melt it, it is preferred to press the resulting molten base material to form a molten mass, whereby a time required for the cooling of the molten mass to the solid can be delayed since the surface area of the molten mass can become small.
It is also preferred to prepare the molten mass in such a manner that the average fiber length in the molten mass can retain at least 95% of its original length in the base material, whereby the mechanical strength of the fiber-reinforced thermoplastic resin molded product such as a tiptoe core for a safety shoe can be improved.
It is also preferred to carry out the step of forming the molten mass and the step of forming the molded product in such a manner that the average fiber length in the molded product can retain at least 90% of the original length in the base material, whereby the mechanical strength of the fiber-reinforced thermoplastic resin molded product such as a tiptoe core for a safety shoe can be improved.
It is also preferred to use air and/or inactive gas as the heating gas which is passed through the accumulated fiber-reinforced thermoplastic resin base material. Air is preferably used from the cost saving, and inactive gas such as N2, Ar, CO2, etc. can reduce the deterioration caused by the oxidation of the thermoplastic resin.
It is also preferred to control the temperature T of the heating gas to pass through the accumulated fiber-reinforced thermoplastic resin base material within the following formula: T1xe2x89xa6Txe2x89xa6T1+100xc2x0 C., wherein T1 is a melting point of the thermoplastic resin. As a result, the thermoplastic resin can be efficiently melted while the deterioration of the thermoplastic resin is reduced.
Further, a process for producing the fiber-reinforced thermoplastic resin molded product of the present invention can be carried out by using a package containing a fiber-reinforced thermoplastic resin base material, wherein the heating gas is applied to the package to melt it thereby to make an opening or a hole in the package and through the opening the heating gas can be passed to the base material. As a result, it can be carried out to prepare the molten mass by heat-melting the base material and supplying the resulting molten mass to a mold to press-mold it.
According to the present invention using the package, a predetermined amount of the fiber-reinforced thermoplastic resin base material is packed with a thermoplastic resin film, whereby good handling and transportation of said base material can be attained. It is possible to produce a molded product efficiently since a blanking step and a measuring step on site are omitted. Accordingly, on the site where a molding is carried out, it is only required to arrange the package containing a fiber-reinforced thermoplastic resin in a string form or in a tape form into a vessel and the like through which a heating gas is passed. Thus, the fiber-reinforced thermoplastic resin base material packed in the package is kept in its bulkily accumulated state, whereby a step s of scattering and accumulating said base material is not needed on site.
Further, in the present invention, it is possible for a heating gas to pass easily through the gaps in the fiber-reinforced thermoplastic resin base material in its bulkily accumulated state by applying a heating gas to the package to make a hole in the thermoplastic resin film of the package. Thus, said base material can be rapidly and uniformly heat-melted to prepare a molten mass. The resulting molten mass is subsequently press-molded to form a molded product with the high mechanical strength due to long fibers contained, and good outer appearance. Moreover, after said base material having been produced, the heat-melting of the resin can be one time, whereby the cost of energy consumption will be reduced and the work efficiency is improved as well as the heat deterioration of the resin will be lowered.
Moreover, when the bulkily accumulated fiber-reinforced thermoplastic resin base materials are heat-melted to prepare the molten mass, the ends of the molten mass is likely to be over cooled. In the present invention, the ends of the molten mass can be covered with the molten thermoplastic resin film of the package during the heating step, and thus the cooling of the molten mass can be avoided.
As an example of a molded product produced by any one of the processes of the present invention, it is possible to manufacture a tiptoe core for a safety shoe which satisfies the performance of S kind standard as safety shoes stipulated in JIS T 8101. The tiptoe core for a safety shoe, produced by heat-melting the fiber-reinforced thermoplastic resin base material to prepare a molten mass and then to press-mold the resulting mass as mentioned before, has the good outer appearance and the light weight as well as the mechanical strength satisfying the performance of S kind standard as safety shoes stipulated in JIS T 8101 due to the long fibers contained and the small heat deterioration of the resin.
The tiptoe core for a safety shoe of the present invention has preferably at most 35 g as the weight of one piece and at most 4 mm as the thickness of its maximum part. Accordingly, said tiptoe core for a safety shoe is excellent in the weight, the compactness and the mechanical strength.